In the field of civil engineering, foundation piles can be classified into three major categories, a non-displacement pile, a partial-displacement pile, and displacement pile, according to influences on soil by the pile-forming method. A pile body of a foundation pile can be shaped with a uniform section or a variable section. The shape of the pile body and the pile-forming method directly determine the bearing capacity, construction speed, project costs, environment protection of the pile and other issues. During the process of continuous evolution of the pile types and the pile-forming methods of the foundation piles, generation of piles with a variable section greatly increases a side frictional force of the piles, so that the pile length is shortened, the pile diameter is decreased, the number of the piles is reduced, significantly reducing the construction costs. Because caisson piles, which are of the non-displacement type, have many problems in terms of technology, cost, and environment protection, such as low bearing capacity of a single pile and pollution of slurry, it is always pursued in the geotechnical engineering field to develop new pile types and construction methods which are not only environmentally friendly and of high bearing capacity, but also economic, especially piles of reasonable displacement type capable of forming a bore and forming a pile in one step, the reason is that in addition to prominent technical and cost advantages, the construction method of such displacement pile, compared with the non-displacement pile, have advantages in terms of construction benefit and environment protection, such as high construction speed, no slurry pollution, no vibration, and low noises.
Currently, as for displacement piles having branches and forks and being capable of forming a bore and forming a pile in one step, extruding and expanding devices, drill stems, piling machine, and pile-forming methods, such as a three-fork extruded and expanded pile, a branch pile, a thread pile, a spiral extruded and expanded pile, a half-screw pile, rotary extruded caisson pile and the like, are domestically reported in relevant documents. These pile types have their special advancement and inventiveness to some extent, but at the same time also have the following defects and problems.
1. The three-fork extruded and expanded pile and the branch pile:
The three-fork extruded and expanded pile and branch pile (e.g. CN201109909Y and CN102021905A), which are caisson piles with a variable section, have a pile body and forks or plates, and their side resistance and end resistance are greatly improved compared with the soil-withdrawal type of caisson piles with a uniform section, but their pile-forming method is relatively complex, the pile-forming procedure is tedious, including drilling a bore→extruding and expanding→cleaning the bore→measuring the diameter of the bore→placing a reinforcement cage→pouring a concrete, increasing the time and costs for constructing a single pile, wherein in case of the soil layer with a higher underground water level, the phenomenon of bore collapse easily occurs during the construction, moreover, the removed soil to be transported out easily causes environmental pollution. All of subsequent corresponding patents are related to modifications to the extruding and expanding devices to improve efficiency of the extruding and expanding methods, but a bore is still pre-formed before forming a pile in their pile-forming methods.
For example, an integrated long-spiral drilling tool for an extruded and expanded branch pile disclosed in CN201334661Y and similar integrated long-spiral drilling tools for an extruded and expanded branch pile make new improvements on the pile-forming methods of the branch piles. After the drilling tool drills to a designed depth, a supporting branch is formed through extrusion and expansion, the pile construction is completed after the drilling tool is lifted from the ground. Although the pile-forming method becomes easy and quick, it is still needed to remove soil in its construction process, resulting in environmental pollution caused by soil transported out. Moreover, there is loose soil at the pile end, thus the settlement deformation is large. Particularly, during the extruding and expanding process of the drilling tool, an extruding and expanding arm is located at the most unfavorable operation angle when it distracts, the maximum bending moment it bears easily breaks or damages its root portion, moreover, in case of relatively hard soil, it is hard for the extruding and expanding arm to distract merely through a hydraulic cylinder. When multiple hydraulic cylinders are hinged, high control preciseness is required, but it is easy to cause mechanical failure, and the equipment costs and the maintenance costs are high.
2. The thread pile with a variable section:
The thread pile with a variable section (e.g. CN2716480Y and CN102162248A) is a pile of soil-displacement type and a fully-threaded pile, and its construction needs to pre-form a bore with a bore-forming machine.
3. The bidirectional spiral extruded and expanded pile:
The bidirectional spiral extruded and expanded pile (e.g. CN101012649A and CN20218431 U) is a cylindrical pile with a uniform section and belongs to a full soil displacement pile. The advantage of this pile is that soil is compacted by a drilling tool so that a side resistance of the pile is prominently improved, while the disadvantage is that in case of relatively hard soil, it is hard for the drilling tool with a bidirectional spiral extruding and expanding drilling bit to drill forward, making it difficult to form a pile, moreover, the pile formed through its extrusion and expansion has the same outer diameter with that of the drilling tool, therefore the energy consumption is significant.
4. The half-screw pile (also called as a screw pile)
The half-screw pile (also called as a screw pile, e.g. CN1254587C, CN1904225A, and CN101016743U) is a pile of soil-displacement type with a variable section. A pile machine reasonably compacts soil according to sensitivity of the soil, therefore negative effects of extruding soil will not be created. The pile-forming method uses a simple mechanical principle so that a rotational force bearing direction is identical with a rotating direction, which can realize expansion and retraction, the bearing capacity of the formed pile is relatively high. Many inventions of this pile and its technologies have been widely applied in the processing of pile foundation and foundation base in the construction field, therefore this technology can substantially improve the bearing capacity and the construction cost of a single pile, thus good social and economic benefits have been obtained, but it still has its limitations. As a result, it is limited in the application range, moreover, a torque of a drill stem needed after the diameter is increased is also correspondingly increased, greatly increasing the mechanical cost, and the energy consumption is large, which should be overcome.
5. The rotary extruded caisson pile
The rotary extruded caisson pile (e.g. CN101016743A) is a caisson pile of soil-displacement type, with its pile body having a uniform diameter or the pile end having an enlarged head. Its construction is realized by a synchronization technology and a non-synchronization technology in a pile machine control system. It may be applied in a wide range, but still has certain limitations. The enlarged head of its pile end is formed using the principle of an expansion bolt, the drill stem used in construction performs the output through double motors, whose cost is relatively high, moreover, the mechanical movement is relatively complex during outward expansion, which easily causes wearing consumption and should be overcome.